Method and apparatus for a derivative spectrometer

ABSTRACT

The invention comprises a method of adapting derivative spectrometry for use in a downhole environment and addresses problems that are inherent in this environment. Such problems include, but are not limited to, elevated temperatures and scattering from particles residing within dirty fluid samples. Under such conditions, the photometric resolution of a spectrometer decreases at the same time that the need for better resolution increases. The invention improves the resolution by measuring the first derivative of the spectrum. The derivative spectrometer of this invention operates by vibrating a linear variable interference filter back and forth along the plane of the filter or by oscillating a circular variable filter about some angle. The effect is to oscillate the wavelength of light that is received by each photodetector. The photodetector signal can be electronically filtered to reject signals that are not at the oscillation frequency and which do not have a fixed phase relative to it. In a preferred embodiment, a vibrating actuator is the means to achieve the required oscillations about a given wavelength. Derivative spectrometry gives a higher resolution than normal methods of spectrometry. Through improved resolution, it is possible to estimate the contamination percentage of the crude oil in real time. Furthermore, it is possible to determine whether a contamination percentage is leveling off over time. It is expected that high-resolution spectra enable an improved estimation of the percentages of methane (natural gas), aromatic, olefins, saturates, and other crude oil properties.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to U.S. Patent Application number notpresently assigned, entitled “A Method and Apparatus for a HighResolution Downhole Spectrometer” by Rocco DiFoggio, Paul Bergen andArnold Walkow, filed on Jun. 4, 2002. This application is related toU.S. Patent Application number not presently assigned, entitled “AMethod and Apparatus for a Downhole Flourescence Spectrometer” by RoccoDiFoggio, Paul Bergen and Arnold Walkow, filed on Jun. 4, 2002. Thisapplication is related to the U.S. patent application Ser. No.10/119,492 filed on Apr. 10, 2002 by Rocco DiFoggio et al., entitled “AMethod and Apparatus for Downhole Refractometer And AttenuatedReflectance Spectrometer” which is hereby incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the field of downholesampling and in particular to derivative spectrometry in a downholeenvironment.

[0004] 2. Summary of the Related Art

[0005] Oil companies take samples from potential hydrocarbon-bearingformations to determine a formation's propensity to producehydrocarbons. Oil companies desire the most accurate measure of samplecontamination percentage in real time as they are pumping fluid from aformation so that they can decide when to divert a sample being pumpedto a sample collection tank. As formation fluid is pumped from theformation, the percentage of filtrate contained in the formation fluidsample diminishes in the pumped fluid. Thus, an oil company typicallypumps until a pure sample, relatively free of filtrate can be obtainedin order to accurately appraise the hydrocarbon producing potential ofthe surrounding formation. The oil company does not, however, want topump unnecessarily long and waste very expensive rig time. Conversely,they do not want to pump too little and collect a useless sample, whichis full of contaminants and does not reflect the properties of theformation fluid. If the contamination of filtrate contained in thesample is more than about 10%, the sample may be useless for itsintended purpose. Moreover, it may not be discovered that the sample isuseless until the sample is retrieved at the surface, making a returntrip downhole necessary to collect another sample. In such cases, thePVT properties indicative of formation and formation fluid propertiesthat are measured in the lab cannot be corrected back to true reservoirconditions because of excessive contamination. It is therefore desirableto perform sample contamination measurements downhole. One method ofinvestigation is to use a spectrometer to perform optical measurementson the fluid samples collected in a downhole environment.

[0006] Numerous factors can affect downhole spectrometer measurements.In the downhole environment, photodetectors operate at high ambienttemperatures and thus are very noisy and produce a substantiallydiminished signal. Also, contaminated samples consisting of flowingstreams of crude oil containing scatterers such as sand particles orbubbles tend to add noise to the system. These scatterers cause theoptical spectrum to momentarily “jump” up (get darker) as they passthrough the sample cell. At high concentrations, these scatterers causethe measured spectrum to move or jump constantly. To first order, theeffect of the scatters is just a momentary baseline offset. An operatorcan greatly improve the signal-to-noise ratio of a downhole spectrometerby modulating the wavelength of light and using a lock-in amplifier.Thus, there is a need for a spectrometer that operates in a downholeenvironment and diminishes the effects of the scatterers and theassociated offsets.

[0007] Spectrometers typically disperse white light into constituentcolors. The resulting rainbow of colors can be projected through asample to be analyzed and onto a fixed array of photodetectors whichsense light projected though the sample. Alternatively, by rotating adispersive element (i.e. grating, prism), the rainbow can bemechanically scanned past a single photodetector one color at a time. Ineither case, an operator can obtain a sample's darkness versuswavelength, in other words, the sample's spectrum.

[0008] Photodetectors and their amplifiers always have some thermalnoise and drift, which limit the accuracy of a spectral reading. Astemperature increases, noise and drift increase dramatically higher atthe same time that photodetector signal becomes significantly weaker. Ifan operator oscillates the wavelength (color) of light about some centerwavelength, then the operator can reject most photodetector andamplifier noise and drift by using an electronic bandpass filter thatpasses only that electrical frequency at which the wavelength of lightis being oscillated. The operator can further reject noise by using aphase-sensitive (“lock-in”) amplifier that not only rejects signals thathave the wrong frequency but also rejects signals that have the correctfrequency but do not have a fixed phase relationship (indicative ofnoise) relative to the wavelength oscillation. A lock-in amplifier canimprove signal to noise by as much as 100 db, which is a factor of10^(100 db/10) or 10 billion.

[0009] The output of the lock-in amplifier used in this procedure isproportional to the root-mean-square (RMS) amplitude of that portion ofthe total signal, which is at the same frequency and has a fixed phaserelationship relative to the optical frequency being observed. The morethat the darkness of the sample changes with color, the larger this RMSvalue will be. Thus, the output of lock-in amplifier for a system withan oscillating-wavelength input is proportional to the derivative of thespectrum (with respect to wavelength) at the center wavelength of theoscillation.

[0010] A spectrometer based on an oscillating-wavelength and a lock-inamplifier can be used to obtain high accuracy spectral measurements asdescribed in the related art below. U.S. Pat. No. 4,070,111 entitledRapid Scan Spectrophotometer, by Harrick, Jan. 24, 1978 discloses aspectrophotometer capable of rapid spectral scanning by mounting a lowinertia reflective grating directly on the output shaft of agalvanometer-type optical scanner, and sweeping the beam dispersed fromthe grating across a spherical mirror and after reflection there fromacross a beam exit slit. The invention also describes rapid wavelengthswitching for a laser spectrometer.

[0011] U.S. Pat. No. 4,225,233 Rapid Scan Spectrophotometer, by OganSep. 30, 1980 discloses a spectrometer capable of providing apredetermined wavelength of output light in accordance with a controlvoltage signal applied to a scanning element. The scanning elementlocated at the grating image of the spectrometer is a small mirrorattached to the rotor of a galvanometer. The angular position of thegalvanometer is accurately controlled by a closed-loop electroniccontrol. The spectrum reflected from the mirror is passed through a slitto provide the output light of a predetermined wavelength. Selection ofthe waveform of the control signal allows the spectrometer to beoperated as a dual wavelength spectrometer, to use a linear wavelengthscan, or other wavelength scan patterns for absorbance analyses of asample.

[0012] U.S. Pat. No. 4,264,205 Rapid Scan Spectral Analysis SystemUtilizing Higher Order Spectral Reflections Of Holographic DiffractionGratings, Landa Apr. 28, 1981 And U.S. Pat. No. 4,285,596 HolographicDiffraction Grating System For Rapid Scan Spectral Analysis, Landa, Aug.25, 1981 discloses an improved optical system for rapid, accuratespectral analysis of the reflectivity or transmissivity of samples. Aconcave holographic diffraction grating oscillating at high speedprovides a rapid scanning of monochromatic light through a spectrum ofwavelengths. The rapid scan by the grating enables the reduction ofnoise error by averaging over a large number of cycles. It also reducesthe measurement time and thus prevents sample heating by excessiveexposure to light energy. A filter wheel is rotated in the optical pathand is synchronous with the grating.

[0013] U.S. Pat. No. 4,968,122 Galvanometer Gimbal Mount, Hlousek et.al., Nov. 6, 1990 discloses an improved mounting in which a rotatingdiffraction grating assembly directly connects the grating to thegalvanometer that rotates the grating. The galvanometer isgimbal-mounted on a plate so that its position, and that of the grating,can be adjusted so that the plane of dispersion of the grating passesthrough a desired point when the grating is rotated.

[0014] U.S. Pat. No. 4,969,739 Spectrometer With Direct Drive High SpeedOscillating Grating, McGee, Nov. 13, 1990 discloses an optical gratingoscillating at a high rate to scan a narrow wavelength band of lightthrough the spectrum dispersed by a grating. The grating is connectedintegrally with the rotor of a motor, which is energized to oscillateits rotor between selected limits. High-speed oscillation is achieved bydriving the motor with a pulse modulator having a duty cycle controlledby the motor speed. The direction that the motor is driven is controlledby the polarity of the pulse-modulated signal applied to a winding ofthe motor. The limits of the oscillation of the grating and the rate ofrotation of the grating between the limits are selectively variable.

[0015] U.S. Pat. No. 5,488,240 Apparatus And Method For Rotating AnOptical Element Using A Moving Coil In A Constant Magnetic Field,Hlousek et al., Jan. 30, 1996 discloses an apparatus and method forrotating an optical element, such as a diffraction grating or mirror,utilizing a moving coil actuator and an optical encoder to provideprecise element position control. The moving coil actuator, which iscoupled to the optical element, is comprised of a coil immersed in amagnetic field created by a pair of magnets. Current flowing in the coilwindings causes the coil, and ultimately the optical element, to rotate.An optical encoder monitors the rotation of the element and providesrotation signals representative of the instantaneous element position toan actuator control circuit. The actuator control circuit phase shiftsthe rotation signals and compares the phase shifted rotation signals toa desired reference signal to generate position and velocity errorsignals.

[0016] The grating and, possibly, additional optical elements direct thelight to the sample or target of interest. The angular displacement ofthe diffraction grating relative to the incoming light beam can beclosely correlated with the individual wavelengths or range ofwavelengths at which the sample is to be analyzed. By controlling theangular rotation and position of the diffraction grating, a range ofwavelengths can be scanned at a known rate over a known time intervaland, consequently, the individual wavelengths can then be distinguishedas a function of time.

[0017] U.S. Pat. No. 5,981,956 Systems And Methods For Detection OfLabeled Materials, Stem, Nov. 9, 1999 discloses a reciprocatingradiation direction system comprising a mirror selected from one of agalvanometric mirror, angularly oscillating mirror, or a rotatingpolyhedral mirror for scanning a focused excitation radiation across asurface of a substrate at a rate of at least 20 image lines/second.Labeled targets on a support synthesized with polymer sequences at knownlocations can be detected by exposing marked regions of sample toradiation from a source and detecting the emission there from, andrepeating the steps of exposition and detection until the sample iscompletely examined.

[0018] U.S. Pat. No. 5,963,320 Active spectrometer, Brooks, et. al. Oct.5, 1999 discloses a grating spectrometer employing digital control of anoscillating component (a mirror) and phase-locked digital recording ofthe intensity profile within the narrow spectral domain defined by anoscillation frequency. Flexible choice of oscillation frequency permitsmeasurement in a quiet region of the noise spectrum. Reference waveformsacquired with the same instrument can be stored and later used tode-convolute a more complex spectrum. The use of multiple detector/slitcombinations along a Rowland circle makes the spectrometer sensitive tospecific atomic elements. A claim is made for an apparatus for providingone or more electrical signals representing a measurement of spectralsimilarity between an emission spectrum from a light source and areference spectrum. It comprises an optical instrument that spectrallydisperses an optical signal, a driver that induces relative movement inthe dispersion direction between the optical signal's imaged componentsin the image region and the template located in the image region, and aplurality of electro-optical sensors.

[0019] F. Vogt, U. Klocke, K. Rebstock, G. Schmidtke, V. Wander, M.Tack, Optical UV Derivative Spectrsocopy for Monitoring GaseousEmissions, Applied Spectroscopy, November 1999, p. 1352. This paper'sFIG. 2 shows an optical grating which is oscillating rapidly as it moreslowly rotates about its axis. That is, the grating rapidly oscillatesabout each wavelength while it more slowly sweeps over a range ofoptical wavelengths. While known devices address derivative spectrometryin a laboratory environment, there is no known derivative spectrometerthat is able to operate under the conditions of a downhole environment.Thus, there is a need for a derivative spectrometer that is able tooperate under the conditions of a downhole environment.

SUMMARY OF THE INVENTION

[0020] The present invention comprises a method of adapting derivativespectroscopy for use in a downhole environment and addresses problemsthat are inherent in this environment. Such problems include, but arenot limited to, elevated temperatures and scattering from particles orother scatterers residing within dirty fluid samples. Elevatedtemperatures reduce the photodetector response for the same light level.Scatterers cause momentary jumps or spikes in the spectra, which, tofirst order, are simply temporary baseline offsets. Such repeatedoffsets make it difficult to obtain quantitative absorbance spectra ofthe pure (scatterer-free) fluid except by taking the first derivativewith respect to wavelength, which removes baseline offsets. The presentinvention improves the photometric resolution by measuring the firstderivative of the spectrum.

[0021] In the present invention, the spectrometer's wavelengthdiscrimination is provided by an optical filter whose color changes fromone portion of the filter to another. By mechanically oscillating such afilter relative to one or more photodetectors, the amplitude of thesignal produced by a photodetector will be proportional to the rate ofchange of the light transmission with wavelength (the first derivativespectra).

[0022] The derivative spectrometer of the present invention can operateby translation or vibration of a linear variable optical interferencefilter, back and forth along the plane of the filter. Alternatively, itcan operate by rotational oscillation of a circular variableinterference filter about some angle. The light can be filtered beforeentering the sample. Alternatively, we can let white light impinge onthe sample, and filter the exiting light. In either case, the wavelengthof light that eventually reaches the photodetector is oscillating. In apreferred embodiment, a vibrating actuator is provided to achieve therequired oscillations about a given wavelength. Derivative spectrometryprovides a higher resolution spectral measurement than normal methods ofspectrometry. Through improved resolution, it is possible to accuratelyestimate the contamination percentage of a crude oil sample in real timeas it is being pumped from the formation. (See co-pending application#xxxx, which is incorporated herein by reference.) Furthermore, thepresent invention enables determination of whether a contaminationpercentage is leveling off over time. High-resolution spectra providedby the present invention enables an improved estimation of thepercentages of methane (natural gas), aromatic, olefins, saturates, andother crude oil properties. The present invention enableshigher-resolution spectral measurements to determine a percentagecontamination for samples and estimation of crude oil properties derivedfrom the samples.

[0023] The present invention directly measures the derivative of thespectra, thereby minimizing baseline offset effect associated withscattering of light by contaminants found in the sample. Scattering,which can cause substantial baseline offsets, is particularlyproblematic for fluids withdrawn from unconsolidated formations. Suchformations produce many fine particles that act as scatterers in asample containing the particles. The present invention enables obtainingthe first derivatives with respect to wavelength, which eliminatesbaseline offsets. These and other features and advantages of the presentinvention will be evident from reading the following description andfigures for the present invention.

BRIEF DESCRIPTION OF THE FIGURES

[0024]FIG. 1 is an illustration of the present invention in a downholeinstallation; and

[0025]FIG. 2 is an illustration of a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0026]FIG. 1 illustrates a preferred embodiment of the present inventiondeployed in a borehole. The present invention is suitable for deploymentin either a wire line, slick line or monitoring while drillingenvironment. FIG. 1 illustrates a preferred embodiment of the presentinvention deployed in a monitoring while drilling operation.

[0027] Turning now to FIG. 1, FIG. 1 is a drilling apparatus accordingto one embodiment of the present invention. A typical drilling rig 202with a borehole 204 extending there from is illustrated, as is wellunderstood by those of ordinary skill in the art. The drilling rig 202has a work string 206, which in the embodiment shown is a drill string.The drill string 206 has attached thereto a drill bit 208 for drillingthe borehole 204. The present invention is also useful in other types ofwork strings, and it is useful with a wireline, jointed tubing, coiledtubing, or other small diameter work string such as snubbing pipe. Thedrilling rig 202 is shown positioned on a drilling ship 222 with a riser224 extending from the drilling ship 222 to the sea floor 220. However,any drilling rig configuration such as a land-based rig may be adaptedto implement the present invention.

[0028] If applicable, the drill string 206 can have a downhole drillmotor 210. Incorporated in the drill string 206 above the drill bit 208is a typical testing unit, which can have at least one sensor 214 tosense downhole characteristics of the borehole, the bit, and thereservoir, with such sensors being well known in the art. A usefulapplication of the sensor 214 is to determine direction, azimuth andorientation of the drill string 206 using an accelerometer or similarsensor. The bottom hole assembly (BHA) also contains the formation testapparatus 216 of the present invention, which will be described ingreater detail hereinafter. A telemetry system 212 is located in asuitable location on the work string 206 such as above the testapparatus 216. The telemetry system 212 is used for command and datacommunication between the surface and the test apparatus 216.

[0029]FIG. 2 illustrates a preferred embodiment of this invention. Thepresent invention provides a linear variable filter 205, that is, afilter whose color bands 211 or color will change linearly from one endof the filter 205 to the other. In practice, linear variable filters areusually prepared by cutting them from the rim of a circular variablefilter. Both linear and circular variable filters are suitable in apreferred embodiment. In both cases, the present invention provides anoptical filter whose transmission wavelength varies from one portion ofthe filter to another. Thus, a variable optical filter, such as a linearvariable or circular variable filter is used in a preferred embodimentto encounter light from the sample.

[0030] In a preferred embodiment, light 201 from a collimated lightsource 201 is directed so that it is incident upon or shown through asample within sample chamber 203. The linear variable filter 205 isvibrated or reciprocally translated by vibrator 206 at an acoustic rangefrequency (e.g., 20-20,000 Hz) parallel to the plane of the linearfilter while the filter situated above a photodetector array 207. Theacoustic frequency of vibration 206 is used as a reference to track themotion of the linear variable filter and to eliminate all signals thatare not at the same frequency as the reference signal 206 and are notfixed in phase relative to reference signal 206. Light 201 that passesthrough the sample 203 therefore is filtered over a small range offrequencies centered around the optical frequency. In a preferredembodiment of the invention, a vibrating actuator 206, for example atuning fork or a piezoelectric actuator vibrates the linear variablefilter 205.

[0031] In a preferred embodiment, a piezoelectric element alone, withouta tuning fork 206, is a provided as a vibration source. The addition ofa tuning fork still requires provision of a external exciter such as anelectromagnetic coil or a piezoelectric element anyway. Thepiezoelectric element can be excited directly by an alternating electriccurrent. Furthermore, it is noted that in a preferred embodiment, inorder to obtain a sufficiently-recoverable signal amplitude, theamplitude of vibration should be 50-100% of the distance between thecenters, referred to as the “pitch” of adjacent photodetector elements207 in the photodetector array 210. The pitch is typically 25 to 200microns. A single piezoelectric transducer typically would provideinsufficient vibration amplitude, typically only 1-2 microns for theintended purpose of the present invention. Therefore, the preferredvibration source is a piezoelectric actuator, comprising either a benderplate, having up to 2 mm amplitude motion/deflection range or a stack ofpiezoelectric transducers, having up to 100 microns amplitude of totalmotion.

[0032] Each photodetector comprising the array 210 is connected to alock-in amplifier 213, preferably through a low-gain preamplifier, whichacts as a buffer to a multiplexer and a multiplexer 208. The lock-inamplifier output for all photodetectors in the array is plotted againstwavelength, thus, it is possible to obtain a first derivative of thespectrum with respect to wavelength. The reference signal 209 as well ascontrol and processing for the present invention are provided byprocessor 212. Processor 212 includes memory and input/outputcapability.

[0033] Obtaining the first derivatives with respect to wavelengthsubstantially eliminates baseline offsets. Thus, the present inventioncan provide real-time oil-based mud contamination percentage. Accuratesample contamination percentage is a highly sought after sampleparameter that oil companies require for accurate formation productivityassessment. Simulation studies show that the present invention couldenable accurate correlation of the percentages of oil-based mudcontamination, regardless of the crude oil type or the filtrate type, tohigh-resolution spectra over the fundamental hydrocarbon band region(3125-2855 cm−1) and, by inference, could provide similar correlation tothe overtones (1550-1800 nanometers) of these fundamentals. Thecorrelation is provided by a neural network or chemometric derivedequations, discussed below, which are implemented either in processor212 or by a processor on the surface (not shown).

[0034] The present invention provides high-resolution spectralmeasurements that are much more accurate and also provides robustcorrelation equations for estimating the percentages of methane (naturalgas), aromatics, olefins, saturates, and other crude oil propertiesthrough chemometrics or a neural network. These correlation equationscan be independent of the crude oil or filtrate involved.

[0035] In a preferred embodiment, the present invention uses chemometricderived equations or a neural network to determine the amount ofaromatics, olefins, saturates and contaminants in a sample analyzed bythe present invention based on spectral measurements. In known samplingtechniques there is no direct measurement of a percent or level ofcontamination in a sample. The present invention provides a training setof known samples and utilizing chemometrics enables a computer todetermine a mathematical expression for a percentage of aromatics,olefins, saturates and contaminants based on the spectrum measured for asample. Chemometrics also eliminates the need to know what each spectralpeak represents and how much a particular peak overlaps another peak.For example, the present invention can be utilized to determine apercent of contaminants based on a chemometric formula derived from aset of known samples for which the percentages of aromatics, olefins,and so on, have been measured by independent means. The training set canalso be used to train a neural network to predict or determine thepercent of aromatics, olefins, saturates and contaminants present in asample.

[0036] The foregoing example of a preferred embodiment is intended forexemplary purposes only and is not intended to limit the scope of theinvention, which is defined by the following claims.

What is claimed is:
 1. A downhole spectrometer comprising: a sondecontaining a spectrometer for traversing a bore hole, the spectrometercomprising: a collimated light source for illuminating a sample; asample chamber for containing the sample; a variable filter having a anaverage wavelength centered about an absorption peak of the sample, toencounter light from the sample; a vibrating actuator attached to thevariable filter for moving the variable filter; and a photodetectorarray for sensing light passing through the variable filter.
 2. Thespectrometer of claim 1, further comprising: a multiplexor attached tothe photodetector array for multiplexing individual photodetectoroutputs.
 3. The spectrometer of claim 1, a vibration reference signalwhich is fixed in phase relative to the vibrating actuator; and
 4. Alock-in amplifier for amplifying that portion of the output of eachelement of the photodetector array which is at the vibration frequencyand in phase with the vibration reference signal.
 5. The spectrometer ofclaim 1, further comprising: a neural network for predicting a propertyof the sample from the measured spectrum.
 6. The spectrometer of claim1, wherein the vibration actuator further comprises a piezoelectricdevice.
 7. The spectrometer of claim 1, wherein the vibration actuatorfurther comprises a stack of piezoelectric devices.
 8. The spectrometerof claim 1, wherein the vibration actuator further comprises a benderdevice.
 9. The spectrometer of claim 1, further comprising: anelectronic band pass filter centered about a chemical absorption peak ofthe sample which rejects electrical noise outside the band of theelectronic band pass filter.
 10. The apparatus of claim 1, furthercomprising: an amplitude of vibration equal to a substantial fraction ofthe distance between centers of adjacent photodetectors.
 11. Theapparatus of claim 1, further comprising: a plot of spectrum versuswavelength to obtain a first derivative of the spectrum with respect towavelength.
 12. The spectrometer of claim 1, further comprising: achemometric equation for predicting a property of the sample from themeasured spectrum.
 13. The spectrometer of claim 1, wherein thevibration actuator further comprises a piezoelectric device.
 14. Amethod for downhole spectrometetry comprising: traversing a boreholewith a sonde containing a spectrometer; containing a sample in a samplechamber; illuminating the sample with a collimated light source; passinglight from the sample through a variable filter; moving the variablefilter; and sensing light passing through the variable filter with aphotodetector.
 15. The method of claim 13, further comprising:multiplexing individual photodetector outputs.
 16. The method of claim13, further comprising: amplifying that portion of the output of thephotodetector which is fixed in phase with respect to the vibratingactuator.
 17. The spectrometer of claim 1, further comprising:predicting in a neural network, a property of the sample from themeasured spectrum.
 18. The method of claim 13, further comprising:vibrating the variable filter with a piezoelectric device.
 19. Themethod of claim 13, further comprising: vibrating the variable filterwith a stack of piezoelectric devices.
 20. The method of claim 13,further comprising: vibrating the variable filter with a bender device.21. The method of claim 13, further comprising: aligning the variablefilter relative to the photodetector so that the average wavelengthstriking the photodetector corresponds to a chemical absorption peak forthe sample; and Scanning in wavelength over the chemical absorption peakby mechanically oscillating the variable filter.
 22. The method of claim13, further comprising: Vibrating the linear variable filter by anamplitude which is a substantial fraction of the distance betweencenters of adjacent photodetectors.
 23. The method of claim 13, furthercomprising: plotting the spectrum versus wavelength to obtain a firstderivative of the spectrum with respect to wavelength.
 24. The method ofclaim 13, further comprising: predicting a property of the sample fromthe measured spectrum from a chemometric equation.
 25. The method ofclaim 13, further comprising: exciting a piezoelectric device to vibratethe variable filter.